The present invention relates to a novel method for synthesizing a cDNA and a kit used for the method, which are useful in a field of genetic engineering.
Analysis of mRNA molecules derived from various genes is very important in order to elucidate biological phenomena. Discovery of an RNA-dependent DNA polymerase, so-called a reverse transcriptase, from a retrovirus has enabled a reverse transcription reaction in which a cDNA is synthesized using an RNA as a template. As a result, methods for analyzing mRNA molecules have made rapid progress. Then, the methods for analyzing mRNA molecules using a reverse transcriptase have now become indispensable experimental methods for studying genes. Furthermore, these methods, which have been applied to cloning techniques and PCR techniques, have become indispensable techniques not only for studying genes but also in wide variety of fields including biology, medicine and agriculture.
However, the conventional reverse transcription methods had many problems such as interruption of a cDNA synthesis reaction, inability to synthesize a long cDNA, low fidelity, and damage of a template RNA in the course of reaction due to long time required for the reaction.
It is considered that the interruption of the cDNA synthesis reaction is due to the secondary structure formed by the RNA as a template. Optimal reaction temperature for a reverse transcriptase from a retrovirus is low. RNAs form complicated secondary structures during the reaction at the temperature. Then, the cDNA synthesis reaction is interrupted at the sites of such secondary structures. A method in which a heat-resistant reverse transcriptase is used has been proposed in order to solve the above-mentioned problem. However, the reactivity of this method is not satisfactory. In addition, reverse transcriptases have not been known to exhibit a proofreading activity during a reverse transcription reaction, and a method for synthesizing a cDNA with high fidelity has not been known.
As described above, it was difficult to synthesize a cDNA with high efficiency and with high fidelity according to the conventional methods. Thus, a more efficient method for synthesizing a cDNA has been desired.
The present invention has been made in view of the prior art as described above. The main object of the present invention is to solve the problems associated with the prior art and to provide a method for synthesizing a cDNA with high reaction efficiency and accuracy.
The present invention is outlined as follows. The first aspect of the present invention relates to a method for synthesizing a cDNA, characterized in that the method comprises conducting a reverse transcription reaction in the presence of an enzyme having a reverse transcription activity and another enzyme having a 3xe2x80x2-5xe2x80x2 exonuclease activity.
The second aspect of the present invention relates to a method for amplifying a cDNA, characterized in that the method comprises conducting a gene amplification reaction using a cDNA synthesized according to the method of the first aspect as a template.
The third aspect of the present invention relates to a kit for cDNA synthesis, which contains an enzyme having a reverse transcription activity and another enzyme having a 3xe2x80x2-5xe2x80x2 exonuclease activity.
The fourth aspect of the present invention relates to a kit for amplifying a cDNA by conducting a gene amplification reaction using a cDNA synthesized according to the method of the first aspect as a template, which contains an enzyme having a reverse transcription activity and another enzyme having a 3xe2x80x2-5xe2x80x2 exonuclease activity as well as a reagent for the gene amplification reaction.
The present inventors have found the efficiency of cDNA synthesis and the fidelity are increased by conducting a reverse transcription reaction in the presence of an enzyme having a 3xe2x80x2-5xe2x80x2 exonuclease activity in a cDNA synthesis reaction. Thus, the present invention has been completed.
One of the main features of the method for synthesizing a cDNA of the present invention is that a cDNA is synthesized using an RNA as a template in a reverse transcription reaction system containing an enzyme having a 3xe2x80x2-5xe2x80x2 exonuclease activity.
Examples of the samples containing RNAs which can be used in the method of the present invention include, but are not limited to, samples from organisms such as a cell, a tissue and a blood, and samples that may contain an organism such as a food, a soil and a waste water. The sample may be a preparation containing a nucleic acid obtained by processing the above-mentioned sample according to a known method. Examples of the preparations that can be used in the present invention include a cell destruction product or a sample obtained by fractionating the product, the total RNA in the sample, or a sample in which specific RNA molecules such as mRNAs are enriched.
The RNAs to which the method of the present invention can be applied include, but are not limited to, RNA molecules such as total RNA, mRNA, tRNA and rRNA in a sample, as well as specific RNA molecules (e.g., RNA molecules each having a common base sequence motif, transcripts obtained using an RNA polymerase and RNA molecules concentrated by a subtraction method). Any RNAs for which a primer used for a reverse transcription reaction can be prepared may be used.
The primer used for synthesizing a cDNA from an RNA as a template in the present invention is not limited to specific one as long as it is an oligonucleotide that has a nucleotide sequence complementary to that of the template RNA and that can anneal to the template RNA under reaction conditions used. The primer may be an oligonucleotide such as an oligo(dT) or an oligonucleotide having a random sequence (a random primer).
In view of specific annealing, the length of the primer is preferably 6 nucleotides or more, more preferably 10 nucleotides or more. In view of oligonucleotide synthesis, the length is preferably 100 nucleotides or less, more preferably 30 nucleotides or less. The oligonucleotide can be synthesized using, for example, the DNA synthesizer type 394 from Applied Biosystems Inc. (ABI) according to a phosphoramidite method. Alternatively, any methods including a phosphate triester method, an H-phosphonate method and a thiophosphonate method may be used to synthesize the oligonucleotide. The oligonucleotide may be derived from a biological sample. For example, it may be isolated and prepared from a DNA prepared from a natural sample digested with a restriction endonuclease. In view of synthesis of a cDNA from a template RNA, the concentration of the primer in the reverse transcription reaction mixture is preferably 0.1 xcexcM or more, more preferably 0.5 xcexcM or more. In view of inhibition of the reaction, the concentration is preferably 10 xcexcM or less, more preferably 5 xcexcM or less.
Any enzymes having reverse transcription activities can be used in the present invention as long as they have activities of synthesizing cDNAs using RNAs as templates. However, enzymes having reverse transcription activities at a high temperature (i.e., heat-resistant reverse transcriptases) are preferable for the purpose of the present invention. Examples of such enzymes which can be used include a DNA polymerase from a bacterium of genus Thermus (e.g., Tth DNA polymerase) and a DNA polymerase from a thermophilic bacterium of genus Bacillus. The presence of a manganese ion in a reaction mixture is indispensable for the exertion of the reverse transcription activity of Tth DNA polymerase. The manganese ion is known to reduce the fidelity of a PCR. Thus, it is required to eliminate the manganese ion when a reverse transcription reaction mixture in which Tth DNA polymerase is used is used for a PCR. DNA polymerases from thermophilic bacteria of genus Bacillus do not require the addition of a manganese ion for the exertion of its reverse transcription activity and the removal thereof upon a PCR. In this regard, DNA polymerases from thermophilic bacteria of genus Bacillus are preferable for the present invention. A DNA polymerase from Bacillus caldotenax (hereinafter referred to as Bca DNA polymerase) and a DNA polymerase from Bacillus stearothermophilus (hereinafter referred to as Bst DNA polymerase) are preferable. These enzymes do not require a manganese ion for the reactions. Furthermore, they can be used to synthesize a cDNA while suppressing the formation of secondary structure of the template RNA under high temperature conditions.
Bacillus caldotenax is a thermophilic bacterium having an optimal growth temperature of about 70xc2x0 C. Bca DNA polymerase from this bacterium is known to have a DNA-dependent DNA polymerase activity, an RNA-dependent DNA polymerase activity (a reverse transcription activity), a 5xe2x80x2-3xe2x80x2 exonuclease activity and a 3xe2x80x2-5xe2x80x2 exonuclease activity.
The enzyme may be either an enzyme purified from its original source or a recombinant protein produced by using genetic engineering techniques. The enzyme may be subjected to modification such as substitution, deletion, addition or insertion by genetic engineering techniques or other means. Examples of such modified enzymes include BcaBEST DNA polymerase (Takara Shuzo), which is Bca DNA polymerase lacking its 5xe2x80x2-3xe2x80x2 exonuclease activity, and Bst DNA polymerase, Large fragment (New England Biolabs), which is Bst DNA polymerase lacking its 5xe2x80x2-3xe2x80x2 exonuclease activity. The above-mentioned enzymes lacking their 5xe2x80x2-3xe2x80x2 exonuclease activities can be preferably used in the present invention in particular.
The amount of the enzyme having a reverse transcription activity to be used is not specifically limited. The enzyme may be used, for example, in an amount used in a conventional reverse transcription reaction. The amount of the enzyme having a reverse transcription activity may be increased as compared with that for a conventional method to conduct the cDNA synthesis reaction more efficiently and to shorten the reaction time. For example, when BcaBEST DNA polymerase is used for conducting a reverse transcription reaction in a reaction volume of 20 xcexcl, the amount of the enzyme in the reaction mixture is 0.5 U or more. In view of cDNA synthesis efficiency, it is preferably 22 U or more, more preferably 42 U or more. The activity of a DNA polymerase as described herein is based on the indication for a commercially available enzyme. An activity of incorporating 10 nmol of total nucleotides into an acid-insoluble precipitate in 30 minutes under reaction conditions suitable for the DNA polymerase is defined as 1 U. A DNA as a template and a reaction temperature suitable for each DNA polymerase are used. For example, in the case of BcaBEST DNA polymerase, an activity of incorporating 10 nmol of total nucleotides into an acid-insoluble precipitate in 30 minutes at 60xc2x0 C. using polydeoxy(ATP-TTP) as a template/primer is defined as 1 U.
The method for synthesizing a cDNA of the present invention is characterized in that it comprises conducting a reverse transcription reaction in the presence of an enzyme having a 3xe2x80x2-5xe2x80x2 exonuclease activity. The enzyme having a 3xe2x80x2-5xe2x80x2 exonuclease activity to be used in the present invention is not limited to specific one as long as it has the activity. For example, a DNA polymerase having a 3xe2x80x2-5xe2x80x2 exonuclease can be used. Examples of such enzymes include xcex1-type DNA polymerases such as a DNA polymerase from a bacterium of genus Pyrococcus (Pfu DNA polymerase (Stratagene), Pyrobest DNA polymerase (Takara Shuzo), Deep Vent DNA polymerase (New England Biolabs), KOD DNA polymerase (Toyobo), Pwo DNA polymerase (Boehringer), etc.) and a DNA polymerase from a bacterium of genus Thermococcus (Vent DNA polymerase (New England Biolabs), etc.), and pol I-type DNA polymerases such as a DNA polymerase from Escherichia coli (polymerase I, Klenow fragment, etc.) and a DNA polymerase from a bacteriophage (T4 DNA polymerase, etc.). Preferably, an xcex1-type DNA polymerase which exhibits a strong 3xe2x80x2-5xe2x80x2 exonuclease activity is used. The pol I-type DNA polymerase or the xcex1-type DNA polymerase refers to a series of enzymes classified on the basis of the amino acid sequence homology. The features of the amino acid sequences are described in Nucleic Acids Research, 15:4045-4657 (1991).
For the purpose of the present invention, an enzyme that acts on a 3xe2x80x2-terminus of a DNA hybridized with an RNA is used. Furthermore, an enzyme that exhibits a 3xe2x80x2-5xe2x80x2 exonuclease activity at a high temperature is preferable. In this regard, an xcex1-type DNA polymerase derived from a hyperthermophilic archaebacterium is preferable. The xcex1-type DNA polymerase derived from a hyperthermophilic archaebacterium is exemplified by a DNA polymerase from a bacterium of genus Pyrococcus.
A cDNA can be amplified by conducting a nucleic acid amplification reaction using the cDNA obtained according to the method described above as a template. Although it is not intended to limit the present invention, for example, a polymerase chain reaction (PCR) is used as the nucleic acid amplification reaction. The enzyme to be used for the PCR is not limited to specific one. A DNA polymerase conventionally used for a PCR can be used. The cDNA obtained according to the method as described above is synthesized from the template RNA with high fidelity. It is desired to conduct the PCR with high fidelity in order to reproduce the sequence from the RNA as accurately as possible. Thus, a DNA polymerase having high fidelity such as an xcex1-type DNA polymerase from a thermophilic archaebacterium is preferably used for the cDNA amplification of the present invention. When a heat-resistant DNA polymerase having a 3xe2x80x2-5xe2x80x2 exonuclease activity (e.g., an xcex1-type DNA polymerase from a thermophilic archaebacterium) is used in the cDNA synthesis step, the same enzyme can be also used in the PCR step. Fidelity of a cDNA synthesis/amplification reaction can be determined according to a modification of the method of J. Cline et al. (J. Cline et al., Nucleic Acids Research, 24:3546-3551 (1996)).
The enzyme may be either an enzyme purified from its original source or a recombinant protein produced by using genetic engineering techniques. The enzyme may be subjected to modification such as substitution, deletion, addition or insertion by genetic engineering techniques or other means. A DNA polymerase having a 3xe2x80x2-5xe2x80x2 exonuclease activity exhibits a proofreading activity which is effective in eliminating a base erroneously incorporated in a synthesized cDNA even if an RNA is used as a template. Thus, it can be used to synthesize a cDNA with high fidelity. In view of cDNA synthesis efficiency, the amount of the enzyme (in terms of polymerase activity) to be used in a reaction volume of 20 xcexcl is preferably 1 U or less, more preferably 0.5 U or less. In view of the proofreading activity, the amount is preferably 0.01 U or more, more preferably 0.02 U or more.
By using the method of the present invention, the amount of synthesized cDNA can be increased, a longer cDNA can be synthesized, and a cDNA can be synthesized with higher fidelity as compared with the conventional reverse transcription methods. Furthermore, by combining the method of the present invention with related techniques such as cloning techniques and PCR techniques, it is possible to prepare a cDNA library or to conduct an RT-PCR more efficiently as compared with conventional methods, thereby allowing more accurate analysis of an mRNA of which the analysis by a conventional method was difficult due to the low expression level.
The kit for cDNA synthesis of the present invention is a kit to be used for the method for synthesizing a cDNA as described above. It is a kit for synthesizing a cDNA with high fidelity and with high efficiency. The kit is exemplified by one that contains the enzyme having a reverse transcription activity and the enzyme having a 3xe2x80x2-5xe2x80x2 exonuclease activity as described above. The kit may contain a reaction buffer to be used for cDNA synthesis reaction using the enzyme described above, nucleotides and other reagents. cDNA can be synthesized efficiently, with high fidelity and readily by using such a kit.
The kit for amplifying a cDNA of the present invention is a kit to be used for the method for amplifying a cDNA as described above. It is a kit for amplifying a cDNA with high fidelity and with high efficiency. The kit is exemplified by one that contains the enzyme having a reverse transcription activity and the enzyme having a 3xe2x80x2-5xe2x80x2 exonuclease activity as described above as well as a reagent for conducting a gene amplification reaction. When a PCR is used as a gene amplification reaction using a cDNA as a template, reagents to be used for the amplification reaction include a heat-resistant DNA polymerase, a reaction buffer, nucleotides and other reagents. cDNA can be amplified efficiently, with high fidelity and readily by using such a kit. When a heat-resistant DNA polymerase having a 3xe2x80x2-5xe2x80x2 exonuclease activity (e.g., an xcex1-type DNA polymerase from a thermophilic archaebacterium) is used in the cDNA synthesis step, the enzyme can be also used in the PCR step as a DNA polymerase for the amplification reaction as described above.